Electron discharge device



May 19, 1959 A. A. RoTow ELEcTaoN DISCHARGE DEVICE Filed April 29.A 1955.lwlmwllllllll INVENTOR. l 1 v ruilnldzv fram/ff United States Patent lCF 2,887,594 ELEcTRoN DISCHARGE DEVICE Alexanderli. Rotow, Lancaster,assignor to Radio yCorporation of America, a corporation of DelawareThis invention relates to television pickup, or camera tubes. Inparticular this invention relatesto an improved image orthicon type oftelevision camera tube.

n image orthicon .tube-normally includes, within an evacuated envelope,an electron gun for producing an kelectron beam. The electron beamhisdirected upon one side .of -a storage target.v v.Within theother end of`the envelope is a photocathode the photoemission from which is directedonto the other side of the target to produce charge patterns on thetarget that are discharged by the electron beam to provide outputsignalsfrom the tube.

When utilizing an image orthicon type of camera tube, there areinstances when it is desirable to obtain greater resolution ofthe scenebeing televised than has been possible heretofore. It is known that theresolution of the image orthicon type of vtube is limited primarily bythe size of the targetrrather thanYthe-size ofthe photocathode. Onesolution to this resolution problem has been to utilize an imageorthicon having a target and photocathode of greater diameter. However,whenutilizing photocathodes of larger diameter `it is necessary toutilize a camera'lens system, which includes expensive lenses 'such asthe telephoto and zoomer lenses, of a size to match thelargerphotocathode. Thisincreased size of lens 'system isconsiderablymore expensive than the 'equivalents in the'smalleruniversally used size of lens systems. v

It is therefore an object of this invention to provide an improved imageorthicon type of tube having improved resolution.

It is anotherlobjectof this invention to provide a new and improvedimage orthicon type of tube which can be used with relativelyinexpensivelens systemsl and one which has improvedresolution. t

These and otherl objects are accomplished in accordance with thisinvention by providing an image orthicon type of pickup tubecomprisingan evacuated envelope. In one end of the envelope is an electron gun andan electron multiplier section having a conventional dynode structure.In the central portion of the envelope there is provided a scanningsection which may comprise deection and focus coils badjacent to thetube as is Well known. Within the other'lend of the envelope there isprovided `an image section including a target and photocathode.

Also, a tube in accordance with this invention includes v a conicalshield and a cylindrical shield both of which are `provided to produce asmooth transition of electric elds between the small photocathode andthe larger target.

The novel features of this invention arepointed out in the appendedclaims. 'l'helinvention itself now be l g 2,887,594 Patented May 19,1959 ice 4 2 described in greater detail in connection with the accompanying single sheet of drawings, wherein:

Figure 1 is a partial sectional view of an image orthicon type of tubein accordance with this invention,

Figure 2 is a fragmentary schematic view showing of Athe magnetic lieldsproduced in the tube shown in Figure 1 and; L

Figure 3 Vis a fragmentary sectional view of an embodiment of theinvention.

Referring speciically now to Figure 1 there is shown a partial sectionalview of an image orthicon type of vtube 10 in, accordance with thisinvention. the image orthicon tube 10 comprises an evacuated envelope 11having an electron gun assembly 12 in one end of the envelope. Theelectron gun assembly 12' includes a conventional heater, cathode,control grid and accelerating electrode structures (not shown) forproducing an electron beam 15. Surrounding the gun-12 is an electronmultiplier structure '19.' An additional accelerating electrode 14 isformed as a wall coating on the inner surface .of the envelope. 'I'heaccelerating electrode 14 is utilized for accelerating the electron beam`15 toward a target electrode 16 enclosed within an image section 13.

. APairs of `horizontal and vertical deflection coils are formed into ayoke structure 17 surrounding the tube envelope 1.1. The deflectioncoils provide fields perpendicular to each other and to the axis of theenvelope` 11 and are connected to appropriate sources -of potential (notshown) for providing frame and line scanning of the electron beam 15over the surface of target 16. Such deilection systems' are well knownand further description thereof is not deemed necessary.

Surrounding the envelope 11 is a focus coil 20 fo providing a magneticeld having lines of force substantially parallel to ytheaxis of thetube'lt] and extending from the end of the electron gun-structure 12 tothe opposite end of the image'sect-ion of envelope 11 as shown.Duringtube operation, theield' produced by focus coil 20 `provides afocusing action on the electrons of beam 15 to bring 'these electrons toawell defined point of focus on the target 16. Also surrounding theenvelope, and adjacent to the gun v12, is .a compensating coil 21 havinga iield perpendicular tothe axis of tube 10. By adjusting coil 21 aroundthe axis of tube 10, any slight miscentering ofthe beam 15, due tomechanical imperfections, can be eliminated; Surrounding the other endof envelope `11 is a divergence coilm22, in accordance with thisinvention, the purpose of `'which will be explained hereinafter. l I

A decelerator electrode 18, which is formed as a ring, is mountedimmediately in front of target 16 and adjacent the scanned surfacethereof.

In the other end of the envelope 11 there is provided an insulatingtarget 16. Thetarget 16 comprises a sheet 28 of insulating material,such as glass, and a ne'me'sh screen 26. Extending fromv vthe outerperiphery of the target 16 and inwardly toward the other end of theenvelope 11 is a target cup 23 that is tubular and has a conicallyshaped end that decreases in diameter as it extends toward the other endof the envelope 11. Adfjacent to the smaller end of the conical targetcup,l or electron shield 24,`is supported an accelerator electrode 25which includes a pair of hollow tubular shaped members 27 and 29. As canbe seen from Figure l, the hollow tubular shaped members 27 and 29 arespacedapart' and electrostatically screen an evaporator filament. 31vfrom the end of envelope 11. Due to the fact that the Ctubular members27 and 29 are spaced apart there is provided a slot between thesemembers through which .an alloymaterial is evaporated from the filament,31 to form a photocathode. 33 on the end wall ofptube 1 0,

,Photocathode 33). isja semi-transparent coating of light Generally,

aanwas with a conductive electrode border 34. One example ofawphotocathode material Vis facesiatedj jlayer` owfl `silver bismuthalloy. 3 r l `During operation of tube. 10,*pliotoelectrons *are image,and are directed'towardtargetl` `These photo- `electrons `travel `at ayhigh velocity along the magnetic lines of force provided by focusingcoil onto the un- 'scanned side .of the glassdarget 16.3"'Tl1'es`e`photoelecsensitive,electron emissive' material-supported onf-the yinner surface of the end ofenvelope 11y and in contact.

structure is shown in Figure 3wherein.the focusing coil 20 is wound witha largerk number ofk turns per unit of area near the photocathode 33than the number of turns around the balance of tube; 10. When thisalternative emitted from `time photocathode 33, response to an y,

trons forma positive charge patternjonthe unscanned f surface of target`16 corresponding to the light density of theoriginal scene projectedonto the photocathode 33. Assuming a condition -whenno 4light isdirectedonto photocathode 33, low energy `electrons `from V`thepriinaryelectron Abeam 15la`nd onthe surface of target 16which is exposed totheelectronbeam; and: drive) this surface 'of target 16 substantially toground, or cathodepotential;

"A`t thisV time, the remaining electrons iny beam 15 *arereflected-back, as a return election beam' 152; toward gun" 12.` f -WhenIa light pattern is directed onto photo- 'cathode' 33, theyphotoelec'trons arey `emitted *from each i elemental` portionY ofphotocathode 33 in -an lamounty proi portional to `the light ,p andshade of the `original Scene.`

The photoelectrons strike 'the' surface of target 16 andinitiatesecbndaryemissiontherefrom.' Dueto the velocity of-thephotoelectrons this `secondary `emission hasfan 'emissionratio greaterthanfnnity from the bombarded areas, which drives -theseareasdn a.positive direction.

Due Ito the thinness of target' 16, with the' re'sultingfhigh struct-ureis utilized the divergence: coil 22 is omitted since its function, isreplaced `by the larger number of turns in the focusing coil 20.

It can be 1 shown electron optically` that, in order to minimizeimage.distortion` in a tube ofthis` type, the diverging or spreadingoiy.the `magnetic lines of force 35, i.e. the magnetic ield which.determinesthe diverging electron paths between the photocathode 33 andthe target '16, shouldr be. a smooth rflow without sharp bends or strongdiversions. same time to obtain` considerable image magnication, thedistance from the photocathode `33 tothe target 16 is greater in tubeslbuilt inaccordance: withthis invention than in standard tubes.

` Electron optical considerations alsshowthat,` in order to minimizeimage; distortion,` the yelectrostatic `lines of force, as Well as the1magneticlines of` force, must be f assrnooth as possible and free ofsharp bends. To achieve 25 accordance with this l invention. A Theconical yshaped this "object .the conio'alshaped shield 24 is`introducedfin f shield 24 provides ia. Smoothtransition `ofelectrostatic `lines efforce from the target 16ste the accelerator elec-`trodeZS. l

, involved .in thisinventioniisy shown below. It should be capacitybetweentthe two sides,th'e saine potential pat- 1 tern is`established onthe` scanned, or electrongun," side f `of target 16. Accordingly, thepotential offthe scanned surface of target 16 will vary frornip'ointtopoinnfrona substantiallyzero volts to severalvolts positivepotential.r When the electron beam 1ISapproaches targetl, at n la-verylow "velocity immediately in Vfront "of target 16,

the gunnlZ. In this manner, as the electron beam is p scanned "over thesurface of target y16, it is reected toward the gun 12 as a modulatedreturn beam15'.` The modulated return beam 15' Vfc` llows`substantallythe same path as the incident beam 15 and strikes `the end `of gun 12which is formed as a dynode electrode and is the first stage` of theelectron multiplier section 19.1 The multiplier section may be any*conventional type where the modulated return beam 15' is converted intooutput signal voltagesfromV a collector electrodein the multiplierelecfrode. In `conventional tubes of, this type, the photoelectron imageis converged fromthe photocathode to the target. `In other words, theuseful area of the target is smaller ,than the useful area of` thephotocathode even though physicallythe photocathode and target Vare'`the Asame size, However, in accordancewith this invention thephotoelectron image is diverged from the photocathode 33 tothe target`plt. Thus, tubes builtin accordance with this invention permit the useof a universally used lens system with the relatively small photocathode33, in 'cooperation With'thehigher resolution of the relatively largetarget `16.

Thediverging photoelectron path coniiguration in accordancewith thisinvention is provided by means `of a div'er'gingcoil 22, around theimage Section13, which produces a magnetic eld `density that is "greateradjacent `not be limited to thisparticlar example. e

An leirample'of specifick dimensionsof the components understood thatthis is merely an example of a tube which has `been successfullyoperated and `the invention should structures in4 the image section,tubesi'n accordance with this invention permit the use oflthe`universally used size of lens system coupled with improved resolution.What is claimed is: l y

l. A camera tube comprising `an"'evacuated envelope, a target cupsupporting atargetwithin said envelope, `a `portion of said target cupextending inwardlytoward the axis of said envelope, ahollow tubularelectrode spaced adjacent to the end of saidlportion of said"target cup,a photocathode spaced "adjacenttoithefother end of said hollow tubularelectrode, thefelctron receiving area ofsaid target "being substantiallylarger "than the electron emissivearea of saidphotocathodafand meansaround said envelope for directlyfocussing an electron image from saidphotocathode onto 'saidtarget andfor enlarging said electron image`from`sad`photocathode ontosaid target. p p

2. A television cantera tube comprising an `evacuated envelope,`electrtm `beam"lproducin'g"means in one end of said envelope, atargetwithin said "envelope Tand-inthe `path of theelectif'bnbeam, atargetfcup-electrode said target being spportedacross an aperture insaidtarget cupelectro'de, a'portion of said target cup electrodeextending outwardly` from adjacent the" peripheryfof said `target toadjacenttiie` wall Aot said envelope, another porend of said otherportion"of"`said""'target cup, a photo- Tosatisfy this conditiomand atthe Inches cathode on said end of said envelope, said target beingsubstantially larger in electron receiving area than said photocathodeis in electron emitting area, a focusing coil around said envelopev andextending at least between 'said target and said photocathode forfocussing an electron image from said photocathode directly onto saidtarget, and a divergence coil around said focusing coil adjacent .tosaid photocathode for enlarging said electron image from saidphotocathode onto said target.

References Cited in the ile of this patent UNITED STATES PATENTS RuskaMar. 21, 1939 Morton Feb. 6, 1940 Lubszynski June 11, 1940 LubszynskiAug. 8, 1950 FOREIGN PATENTS Germany Sept. 17, 1953

